The present invention relates to microwave curable adhesives, and compositions and methods for improving the curing thereof.
Conventionally, thermoplastic and thermoset adhesives are cured using radiant heat or chemical reactions through the use of a catalyst or initiator. In an effort to reduce the cure time often required by these techniques, it has been suggested to use ultrasonic or electromagnetic techniques. See, for example, U.S. Pat. Nos. 3,620,876 to Guglielmo, Sr. et al; 4,219,361 to Sutton et al.; and 5,248,864 to Kodokian. Ultrasonic bonding utilizes acoustical properties of the material to be bonded. Electromagnetic bonding is accomplished by one of three methods: (a) magnetic energy induction; (b) dielectric energy generation; or (c) microwave generation. There is a particular interest in the use of microwave generation in that it permits articles to be bonded by rapidly curing the adhesive.
Heat is generated in materials irradiated with microwaves in accordance with either orientation polarization or equivalent resistance heating. The operative mechanism is dependent upon the operating frequency. Orientation polarization is perhaps the most important mechanism of polarization in the microwave frequency range. Equivalent resistance heating results from the flow of conductive current in the substance. The current is related to electronic conduction and ionic conduction in a material. Equivalent resistance heating is more significant at lower frequencies.
Also known as dipolar polarization, orientation polarization involves the perturbation of the random motion of ionic or molecular dipoles to produce a net dipolar orientation under the direction of an applied electric field. Orientation polarization depends on the internal structure of the molecules and on the molecular arrangement or the structure of the dielectric substance, i.e., the adhesive or substrate. Thus, each material exhibits a specific dielectric behavior (losses) which is dependent upon the range of temperatures and frequencies used during processing. Dielectric loss measurements of a given material over the range of temperatures and frequencies of interest provide the information necessary to select frequencies and to select those frequencies which optimize heating of that material. For most polymeric materials, the orientation polarization loss peak (maximum loss) shifts to higher frequencies as the temperature of the material is increased.
Many adhesives, however, are not susceptible to microwave curing, or if the adhesives are, curing is slow or difficult to control or both. Thus it has been proposed in, for example, U.S. Pat. No. 4,626,642 to Wang et al., to blend electrically conductive fibers into a thermoset adhesive to accelerate cure rates. U.S. Pat. No. 4,906,497 to Hellmann et al. proposes the use of electrically conductive materials (e.g., carbon fibers) to accelerate the heating-up rate by microwaves.
In addition, microwaves, particularly single frequency microwaves, are not always uniformly distributed throughout the microwave oven. Differential heating can result in heating only specific portions of the adhesive resulting in uneven curing. An alternative to single frequency microwaves and this problem of differential heating is to use variable frequency microwaves as is suggested in U.S. Pat. No. 5,321,222 to Bible et al. The use of variable frequency microwaves, however, does not typically overcome susceptibility problems.
Thus there remains a need to provide adhesives which are susceptible to curing using various microwave techniques particularly using variable frequency microwaves.